Heretofore known mixing valve assemblies generally provide for two dissimilar fluids, such as hot water and cold water, to flow into a mixing chamber through adjustable-size passages, and the volume of flow from the mixing chamber to a discharge spout is regulated by a valve. In such assemblies a check member, usually a sleeve of rubber or other elastomer, is provided to shut off both fluids when the volume control valve is closed; but when such check member develops a leak, fluid from one source may leak into the other fluid, e.g. hot water may leak into the cold water line or vise versa. Moreover, such sleeve checks are subject to mechanical and thermal wear, and their elasticity may be altered undesirably by being subject to high temperature differentials.